The subject matter of the present invention is a spray concrete that is easy to work even at a relatively low water-cement ratio, can be sprayed well, and sets and hardens rapidly. These properties are achieved by adding to the spray concrete certain concrete liquefiers and accelerators before it emerges from the spray nozzle.
It is known to use as a starting mixture for the wet-spray process wet mixtures of cement, additives, water, and additives if used, as specified in DIN 18551. This wet mixture is delivered either pneumatically through the spray nozzle to the point of application (thin-stream method) or it is pumped hydraulically to the spray nozzle (solid-stream method), where it is converted by compressed air to a thin stream. In both cases the wet mixture that is applied is in close contact with the water for a certain amount of time, which in practice can amount to as much as 90 minutes, and has already reacted with it.
It is also known to add an accelerator to a spray concrete so that the concrete will set as rapidly as possible after it is sprayed on. The accelerator is added just before spraying, preferably directly as it is used at the point of application. The accelerators include concentrated solutions of alkali aluminates, alkali hydroxides and alkali carbonates, such as those described, for example, in U.S. Pat. No. 4,504,315. The accelerators cited therein exercise their full action in the dry spraying process when they are added together with the water (hereinafter also called "mixing water") to the dry-spray concrete according to DIN 18551. Then they act spontaneously on the cement to accelerate setting in the mixing water at the spray nozzle directly at the point of application of the spray concrete. If, however, the spray concrete has already been mixed with the mixing water for a longer period of time and has already reacted, as is the case in the wet-spray process, the accelerators described in U.S. Pat. No. 4,504,315 could no longer produce their optimum effect.
The sole use of liquid accelerators in the wet spray process can cause the consistency of the spray concrete after emerging from the nozzle to be too low, so that the concrete adheres insufficiently especially to vertical walls and overhead areas. Increasing the consistency of the spray concrete by adding lesser amounts of mixing water to obviate this disadvantage is impossible, because then the concrete will no longer be able to be pumped or driven, so that another way of avoiding this disadvantage must be sought.
One known answer to the problem of pumping spray concrete with a low water-cement ratio consists in adding known concrete liquefiers to the concrete. Such additives do increase the readiness of fresh concrete to be pumped and worked, yet they diminish the action of many commonly used accelerators. Thus for example, the solidification accelerating action of the potassium aluminate-base solidification accelerators listed in U.S. Pat. No. 4,504,315 is partially or entirely cancelled by the known concrete liquefiers of technical grade based on alkalinized or neutralized salts of lignin sulfonates. This disadvantage is obviated by using known concrete liquefiers conjunction with water glass. The latter, however, must be added in amounts between 10 and 15% with respect to the cement in order to achieve an adequate hardening action. These amounts, however, have the disadvantage, in turn, that the strength of the set concrete is diminished. The loss of strength can amount to as much as 50% and more.
The problem therefore was to find a spray concrete liquefying and strengthening system that would keep the spray concrete easy to work at the lowest possible water-cement ratios, so that it could easily be pumped in the spray equipment, and at the same time produce the quickest possible stiffening, setting and hardening of the concrete after leaving the spray nozzle. The liquefier contained in the system must therefore not have a negative influence on the action of the accelerator.